定向凝固高温合金晶界形貌的分形描述SCIEI

用分形分析的方法定量描述了定向凝固高温合金的树枝晶晶界形貌。结果表明，随着凝固速率的提高，晶界分形维数均增大。还讨论了凝固合金系统熵与晶界分形维数之间的关系。     

70%Si-Al合金的热性能及其分形描述

采用分形方法探讨70%Si-Al热沉材料的典型热性能与显微组织之间的关系,针对不同Si相尺寸的分形特征进行描述。结果表明:70%Si-Al合金的显微组织具有自相似的界面形态和典型的分形特征,使用分形维数能够直观表示出70%Si-Al合金中两相间的界面性质;计盒维数法和质量维数法2种不同的分形描述方法反映的规律相同;Si相平均尺寸为75μm的试样比Si相平均尺寸为45μm的试样的分形维数更小;随着温度的升高,分形维数较小的合金热导率较大,热膨胀系数增加的幅度却减小。     

P，Zr，B对定向凝固IN738合金组织和偏析的影响SCIEI

用不同凝固速度下定向凝固和定向凝固加快淬的方法研究了Ｐ，Ｚｒ，Ｂ对定向凝固ＩＮ７３８合金组织和偏析的影响发现随磷含量的增加，合金固液两相区宽度及温度区间显著增加；凝固前沿低熔点液相区增大；树枝晶组织发达，二次晶变细、变多；固液界面表面积增加。发现只加Ｐ与同时加Ｐ，Ｚｒ合金的溶质原子偏析规律有差异。Ｐ，Ｚｒ，Ｂ促进合金中（γ＋γ′）共晶形成，在其前沿有（γ＋硼化物）共晶析出。     

熔体-结晶相固-液界面能的研究进展

结晶相的凝固特性决定其凝固后的组织和性能,要对结晶相凝固特性与过程进行准确表征与有效控制,就必须知道熔体-结晶相固-液界面能的准确数据。本文结合作者长期以来对熔体-结晶相固-液界面能的研究工作,论述了熔体-结晶相固-液界面能实验测定和理论研究方面的相关进展。通过对不同温度下固-液界面能实验测定结果的分析比较,发现熔体-结晶相的固-液界面能随温度的降低而减小;找出了Spaepen固-液界面能模型与实验测定结果不相符的原因;提出基于固-液界面结构的固-液界面能理论模型及用其确定固-液界面能的方法。结果表明,当前模型对固-液界面能的预测结果不仅与熔点温度下实验值和模拟值相吻合,而且也与过冷温度下实验值和模拟值相吻合。     

纵向磁场对Co-8.8Al-9.8W-2Ta钴基高温合金定向凝固组织的影响

对Co-8.8Al-9.8W-2Ta钴基高温合金进行了外加纵向静磁场中定向凝固试验。考察了磁场强度对Co-8.8Al-9.8W-2Ta合金凝固组织一次枝晶间距的影响。结果显示,在合金熔体固-液界面前沿的温度梯度为50 K/cm时,在磁场中定向凝固的Co-8.8Al-9.8W-2Ta合金的一次枝晶间距先增大再减小。这种现象是由于在合金溶体固-液界面前沿由磁场诱发的热电磁对流(TEMC)所致。     

由实验相图和数值模型对Al-1.5Cu-3Zn合金凝固路径的预测

使用DTA和LMC定向凝固淬火技术分别确定了Al-Cu-Zn合金富铝角的液相面和固／液界面溶质分凝因数。基于一维平板模型的枝晶几何形状描述，使用数值模型计算了Al-1．5Cu-3Zn合金凝固过程中的液相浓度和平衡温度。发现在平衡凝固条件下，液相在α相区内结束凝固，Scheil非平衡凝固条件下的凝固过程在单变量线上结束，样品中初生晶相的含量为97．4％，非平衡共晶含量为2．6％；同时分析了液相混合条件对凝固路径的影响。     

Sn对Al-Pb偏晶合金凝固过程及组织的影响

研究了添加第三组元Sn对定向凝固Al-Pb合金组织的影响.结果表明,添加微量Sn能降低Al-Pb合金液-液相变时两液相间的界面能,促进富Pb相液滴形核和弥散型Al-Pb合金凝固组织的获得;随着Sn添加量的增加,弥散相体积分数增大,液-液相变温度区间和液-液-固三相温度区间之和增大,初生弥散相粒子和偏晶反应弥散相粒子的尺寸均增大;添加Sn促进Al-Pb合金中凝固界面枝晶化,这一方面促进弥散相液滴/粒子沿凝固方向均匀分布,另一方面,促进弥散相液滴/粒子沿定向凝固合金晶界分布.     

铝基四元合金枝晶组织及微观偏析的数值模拟

建立了基于元胞自动机(CA)和热力学相平衡计算引擎(PanEngine)的二维耦合模型,并应用于铝基四元合金枝晶组织和微观偏析的数值模拟.在该耦合模型中,采用CA方法模拟枝晶组织的演变.以固/液界面的平衡液相线温度和实际温度的差值作为枝晶生长的驱动力,同时考虑了固/液界面曲率的Gibbs-Thomson效应.通过求解溶质传输方程获得固/液界面处三种溶质的液相成分,耦合PanEngine获得固/液界面处的平衡液相线温度及三种溶质的平衡固相成分,为提高计算效率,采用预制数据表格的优化策略将CA与PanEngine进行耦合.将Al-4.5Cu-0.5Mg-1Si(质量分数,%)四元合金凝固时的固相分数随温度的变化以及固相分数和固相成分分布关系的模拟结果与Scheil模型和平衡凝固模型的预测结果进行了对比,结果表明,该模型不仅可应用于模拟多元合金中的枝晶生长形貌,而且能对铝基四元合金系凝固的微观偏析进行定量预测.     

MODELING OF DENDRITIC STRUCTURE AND MICROSEGREGATION IN SOLIDIFICATION OF AL-RICH QUATERNARY ALLOYS

建立了基于元胞自动机(CA)和热力学相平衡计算引擎(PanEngine)的二维耦合模型, 并应用于铝 基四元合金枝晶组织和微观偏析的数值模拟. 在该耦合模型中, 采用CA方法模拟枝晶组织的演变. 以固/液界面的平衡液相线温度和实际温度的差值作为枝晶生长的驱动力, 同时考虑了固/液界面曲率的Gibbs-Thomson效应. 通过求解溶质传输方程获得固/液界面处三种溶质的液相成分, 耦合PanEngine获得固/液界面处的平衡液相线温度及三种溶质的平衡固相成分. 为提高计算效率, 采用预制数据表格的优化策略将CA与PanEngine进行耦合. 将Al-4.5Cu-0.5Mg-1Si(质量分数, %)四元合金凝固时的固相分数随温度的变化以及固相分数和固相成分分布关系的模拟结果与Scheil模型和平衡凝固模型的预测结果进行了对比, 结果表明, 该模型不仅可应用于模拟多元合金中的枝晶生长形貌, 而且能对铝基四元合金系凝固的微观偏 析进行定量预测.     

凝固微观组织的多层次模拟

通过引入溶质再分配、溶质扩散、界面能各向异性和界面曲率,构建了描述合金凝固微观组织形态演变的元胞自动机模型.在介观和微观尺度上的模拟结果表明,该模型可有效地解决微观组织形成的多尺度问题.模拟结果清晰地再现了与实测结果相一致的枝晶形态和生长现象.根据分形理论采用分维定量比较了模拟结果和实验结果,两者的计盒维数分别为1.703和1.694,阐述了分维定量表征枝晶形貌的物理含义.模拟结果表明熔体过冷度和树枝晶的计盒维数呈近似抛物线关系.     

FRACTAL ANALYSES OF DIRECTIONAL SOLIDIFICATION BEHAVIOR OF A Ni-BASE SUPERALLOY

The morphology of solid/liquid interface of a directionally solidification process is describedby means of fractal analysis,and the relations among the fractal dimension of the solid/liq-uid interface,solidification rate,dendrite arm space and the phosphorus content of the test al-loys have been given.It was found that the increase of the solidification rate and the phos-phorus content of the test alloy will lead to a increase,following the regularity of exponentialfunction,in the fractal dimension of solid/liquid interface.Furthermore,by combining thefractal theory and the thermodynamic principle with the measured results,it has been provedthat the fractal dimension is not only a kind of simple geometrical parameter used in des-cribing irregular geometry,but also a state function depending the change of solidificationparameters and chemical compositions.     

连续凝固偏晶合金薄带凝固组织进化过程模拟

分析了偏晶合金薄带垂直连续凝固过程,发展了描述该条件下偏晶合金凝固组织演变的数学模型,将计算的温度场和浓度场与凝固组织演变的控制方程相耦合,模拟了Al-5Pb(质量分数．%)合金的凝固过程．结果表明．在固/液界面前存在一过冷区、弥散相液滴在此区间内形核,这些液滴在移向凝固界面的过程中进行扩散长大,随着凝固速度的提高,形核率升高,弥散相液滴的数量密度增大．平均半径减小。     

Phase transformation studies at the solid/liquid interface by directional solidification of Alloy 718

The phase transformation temperatures are especially important in alloy 718 because it requires careful heattreatment cycles. The DTA method is generally used to estimate the characteristic temperatures of phase transformations. However, DTA is an indirect method to confirm phase transformations, and there are always discrepancies in analyzing the DTA thermograms. In this study, the directional solidification and quenching technique has been applied to estimate the phase transformation temperatures near the solid/liquid interface temperatures by measuring temperatures directly during directional solidification. These phase transformation temperatures were confirmed in the quenched solid/liquid interface. Solidification behavior at the solid/liquid interface with solidification rate and temperature gradient were also discussed.     

锌铝合金热型连铸的工艺方法

就热型连铸工艺条件下对具有共晶、包晶和共析转变的5种典型锌铝合金的连续定向凝固进行了研究,找出了各种合金的最佳工艺参数配合范围.研究表明引锭操作是热型连铸工艺的关键环节,型口温度、拉铸速度、冷却条件、合金成分和液位压头对铸锭表面质量有直接影响,型口位置的固液区存在着热和力的平衡.只有通过调节工艺参数,维持固液界面的良好位置,才能拉铸出表面光滑的线材.     

Model for calculation of microstructural development in rapidly directionally solidified immiscible alloys

A model has been developed for the calculation of the microstructural evolution in a rapidly directionally solidified immiscible alloy.Numerical solutions have been performed for Al-Pb immiscible alloys.The results demonstrate that at a higher solidfication velocity a constituteional supercooling region appears in front of the solid/liquid interface and the liquid-liquid decomposition takes place in this region.A higher solidification velocity leads to a higher nucleation rate and ,therefore,a higher number density of the minority phase droplets.A s a result,the average radius of droplets in the melt at the solid/liquid interface decreases with the solidification velcity.     

真空吸铸AZ61A镁合金的亚快速凝固行为研究

设计了真空吸铸急冷法制备镁合金小尺寸薄片铸件工艺装置,实现了镁合金AZ61A的亚快速凝固.研究分析了亚快速凝固条件下镁合金凝固态组织的特点及其亚快速凝固行为,并与常规凝固镁合金进行了比较分析.结果表明:亚快速凝固过程中,极快的冷却速率使得熔体凝固过程中固/液界面推移的速率很快、局部凝固时间很短,抑制了溶质元素扩散,使其很大程度上固溶于基体组织中;镁合金的晶粒尺寸较常规凝固试样有了明显细化,晶粒度只有几个微米,且随着冷却速率的增加,其尺寸不断减小;提高了铸件成分及组织的均匀性,凝固态组织中析出的3-Mg17Al12相极少甚至消失,最终形成以过饱和c-Mg固溶体为主相的亚快速凝固组织.     

Microstructure Evolution of a Ternary Monotectic Alloy During Directional Solidification

A model was developed to describe the microstracture evolution in a directionally solidified ternary monotectic alloy.The directional solidification experiments were carried out on Al-3Pb-lSn(wt%) alloys by using a Bridgman apparatus.The microstracture evolution in the directionally solidified sample was calculated.The numerical results agree well with the experimental ones.It is demonstrated that the nucleation of the minority phase droplets occur at two different positions.One corresponds to the liquid-liquid decomposition,which occurs in front of the solidification interface.The other is at the liquid/solid interface.The nucleation rate of the minority phase droplets at the liquid/solid interface is significantly higher than at the position in front of the solidification interface.The characteristic of the nucleation process leads to a bimodal size distribution of the minority particles in the directionally solidified sample.     

MICROSTRUCTURAL FEATURE OF Al-In MONOTECTIC ALLOY PROCESSED UNDER MICROGRAVITY

The microstructural feature of Al-In monotectic alloy processed under microgravity has beeninvestigated It was found that in the sample cooled in space,there are a lot of In particles inA! dendrite,but no particle has been observed in the sample cooled on the ground.Moreover,the In particles distribute with regularity and the A1 dendrite has an obvious boundary layermarked by a string of particles.Thus,the distinction between them may reflect the character-istics of the solid/liquid interface during solidification under different gravity conditions.     

Numerical simulation of microstructure evolution and microsegregation of U-Nb alloy during solidification process

In this work, a cellular automaton model has been developed to simulate the microstructure evolution of U-Nb alloy during the solidification process. The preferential growth orientation, solute redistribution in both liquid and solid, solid/liquid interface solute conservation, interface curvature and the growth anisotropy were considered in the model. The model was applied to simulate the dendrite growth and Nb microsegregation behavior of U-5.5Nb alloy during solidification, and the predicted results showed a reasonable agreement with the experimental results. The effects of cooling rates on the solidification microstructure and composition distribution of U-5.5Nb were investigated by using the developed model. The results show that with the increase of the cooling rate, the average grain size decreases and the Nb microsegregation increases.     

Effect of strontium and solidification rate on eutectic grain structure in an AI-13 wt% Si alloy

The influence of strontium addition and solidification rate on eutectic grain structure in a near-eutectic AI-Si alloy was investigated. The characteristic temperature of eutectic nucleation (TN),minimum temperature prior to recalescence (TM),and the growth temperature (TG) during cooling were determined by quantitative thermal analysis. All characteristic temperatures were found to decrease continuously with increasing Sr content and solidification rate. Microstructural analysis also revealed that the eutectic grain size decreases with increasing Sr content and solidification rate. Such eutectic grain refinement is attributed to the increased actual under-cooling ahead of the liquid/solid interface during solidification.